Optical coherence tomography (OCT), as employed for imaging through scattering media, with significant applications for medical imaging, is typically based upon the notion of coherently ranging scatterers in a focal volume using a high-bandwidth, low temporal coherence source, such as a superluminescent diode. By measuring the interferometric cross-correlation between a reference beam and the backscattered return beam, the scatterers corresponding to a particular time delay, typically corresponding to a depth in the medium, can be ranged. Techniques employing OCT are surveyed in Bouma et al., Handbook of Optical Coherence Tomography (Marcel Dekker, 2001), which is incorporated herein by reference. Optical coherence microscopy (OCM), described, for example, by Schmitt et al., Subsurface Imaging of Living Skin with Optical Coherence Microscopy, Dermatology, vol. 191, pp. 93-98 (1995) and Izatt, Optical Coherence Microscopy in Scattering Media, Optics Letters, vol. 19, pp. 590-592 (1994), incorporated by reference herein, combines the depth-ranging capability of OCT with confocal microscopy to obtain micron-scale imaging beneath the surface of a scattering medium.
Traditional OCT operates by illuminating the object with a focused, broadband beam. The back-scattered light is collected, and by using interferometric detection, the time delay and therefore the distance along the beam to scatterers inside the object is determined. By scanning the beam through the object, the locations of scatterers in three dimensions can be found. The resolution of OCT in the axial direction, along the direction of propagation of the beam, is determined primarily by the bandwidth of the light source. However, the resolution in the transverse direction is not constant along the beam. At the focus of the beam, the resolution is determined by the focused spot size, but away from the focus, the resolution degrades because the beam is diverging or converging. This loss of resolution is usually assumed to be an inevitable consequence of defocus. Because the depth-of-field or the confocal volume decreases in size as the numerical aperture of the illumination and detection optics increase, improved transverse resolution results, according to current practice, in a smaller range of depths that can be resolved for a typical OCT system unless the focus is mechanically scanned. “Confocal volume,” as used herein, refers to a volume that is essentially in focus, within a specified criterion, at a single relative placement of the sample relative to the optical system.